a. Field of the Invention
The invention relates to a device for manufacturing a double-walled thermoplastic pipe with a connecting sleeve.
b. Related Prior Art
A device of this type is known for example from DE 101 10 064 A1. The device has a number of molds guided on a path, which form a molding tunnel, which has in at least a first portion a corrugated molding wall and in at least a second portion a sleeve recess corresponding to the connecting sleeve. An extrusion head of the device has a first die for the extrusion of a first flexible tube into the molding tunnel and a second die, arranged downstream in the direction of movement of the molds in the molding tunnel, for the extrusion of a second flexible tube. Arranged between the two dies is a first gas channel, which is connected to a first compressed-gas control device for generating a pressure p1 or p2 in the space between the two tubes by the compressed gas emerging from the mouth of the first gas channel. A second gas channel, which opens out downstream of the second die in the direction of movement of the molds of the molding tunnel, is connected to a second compressed-gas control device, in order to generate a pressure p3, lying above atmospheric pressure, on the inner side of the second flexible tube by the compressed gas emerging from the mouth of the second gas channel. The two compressed-gas control devices are controlled in an open-loop or closed-loop manner by means of a control device.
With the device, the first flexible tube is extruded into the molding tunnel. In the first portion of the molding tunnel, the first flexible tube is brought into a corrugated form and, in the second portion of the molding tunnel, it is expanded to form the connecting sleeve. The second flexible tube is extruded into the first flexible tube and pressed against the corrugation troughs of the first flexible tube, so that a composite pipe comprising an outer tube and an inner tube fused with the latter is formed. While the first flexible tube is being brought into the corrugated form and the second flexible tube is being extruded into the first flexible tube, the space between the two tubes is subjected to a pressure p1 lying above atmospheric pressure (the space between the two flexible tubes is referred to hereafter as space A). The pressure p1 is set at such a level that, after the cooling of the flexible tubes fused together at the corrugation troughs, the inner tube is not curved inward or outward between these points. After the cooling of the flexible tubes, it is intended that atmospheric pressure will be established there. Moreover, the pressure p1 produces the corrugated form of the first flexible tube.
If the first flexible tube is to be expanded in the second portions to form the connecting sleeve, the pressure p2 is set in the space A. The pressure p2 must not be too low, otherwise the first flexible tube would not expand, or not expand adequately to form the connecting sleeve. If, on the other hand, the pressure p2 is too great, the first flexible tube is stretched during extrusion, so that it has a thinner wall thickness at the beginning of the formation of the connecting sleeve and a thicker wall thickness at the end of the connecting sleeve.
During the extrusion of the second flexible tube into the first flexible tube which has been expanded to form the connecting sleeve, the second flexible tube is subjected to a pressure p3 above atmospheric pressure from the inside and pressed against the first flexible tube. As a result, it is ensured that fusion of the two flexible tubes is achieved over their full surface area in the region of the connecting sleeve.
Once the connecting sleeve has been formed by the two flexible tubes and the second flexible tube is extruded again against the corrugation troughs of the first flexible tube in a further first portion of the same, the space A is again subjected to the pressure p1.
The closed-loop or open-loop control of the pressures p1, p2 and p3 takes on special significance, since curvatures of the inner flexible tube and/or irregularities in the wall thickness of the connecting sleeve can be avoided to the greatest extent by suitable setting of the pressures. In the case of the device described above, closed-loop control of the pressures p1 and p2, to which the space A is subjected, in particular proves to be difficult. This space is in this case bounded by the extrusion head and the two flexible tubes, it being intended that the first flexible tube either lies against the corrugated molding wall or against the sleeve recess. Since the diameter of the sleeve recess is greater than the diameter of the corrugated molding wall, the volume of the space A depends on the respective stage of the process. Consequently, the volume changes at the beginning and at the end of the manufacture of the connecting sleeve. In particular in the case of large pipe diameters, the changes in volume in the space A influence the pressures p1 and p2 to such an extent that maintaining them imposes very great demands on the pressure control.
Furthermore, the temperature in the space A also influences the pressure prevailing there. The temperature depends on the amounts of heat that are supplied to and removed from the space A. The amounts of heat in turn depend on the surface areas of the space A via which the amounts of heat are transferred. Consequently, the different form of the molds in the first portion (corrugated) and the second portion also influences the pressure prevailing in the space A.